1. Field of the Invention
The present invention relates to a control device for a brushless direct current motor. The control device switches operation of the motor from a start-up-drive mode to a self-controlled-drive mode.
2. Description of Related Art
A brushless DC motor control device which automatically switches its operation from a start-up-drive to a self-controlled-drive has been known hitherto. The control device of this kind operates the motor by imposing PWM (pulse-width-modulated) driving voltage having a duty ratio in a predetermined range on an armature to generate a rotating field therein until the motor reaches a certain speed (start-up-drive). After the motor reached a certain speed, the control device detects a rotor position from a phase of voltage generated by the armature, formulates PWM driving voltage having a phase corresponding to the rotor position and operates the motor by applying PWM driving voltage to the armature (self-controlled drive). Speed control under the self-controlled-drive is performed by changing the duty ratio of the PWM driving voltage. Since the control device of this kind operates the motor based on the phase of induced armature voltage under the self-controlled-drive, there is no need to equip the motor with a separate rotor position sensor, and therefore the structure of the motor can be simplified. Under the start-up-drive, it is preferable to employ a current controlled PWM operation to limit a high current which tends to appear at a low speed. Generally, a carrier frequency in the current controlled PWM is set at a relatively low level to secure a sufficient current shut-off period. On the other hand, under the self-controlled-drive a carrier having a relatively high frequency is used, avoiding an audible frequency band, because a high current does not appear due to a high speed of the motor. However, in the conventional control, a drive synchronism may be lost (loss of synchronism) and the motor may stop at a transition from the start-up-drive to the self-controlled-drive if a drive torque change is large, e.g., a duty ratio change is too small to cover the torque change. If the duty ratio change is too large, then current consumption and a shock at the transition become large.
To cope with the problem mentioned above, a control method to change the duty ratio gradually during the transition period is proposed in JP-A-9-131091. The duty ratio of the PWM driving voltage immediately after the transition to self-controlled-drive is determined according to the duty ratio of the start-up-drive immediately before the transition. Then, the duty ratio is gradually decreased so that a phase difference between the PWM driving voltage and a detected armature voltage becomes within a predetermined range, and the driving mode is switched to the self-controlled-drive after the phase difference becomes sufficiently small. Generally, rotational speed of the brushless DC motor includes a periodical fluctuation when the motor drives a load such as a compressor having a drive torque ripple. The period of such fluctuation is shorter than a period of one rotation of the motor. Therefore, in the conventional device, there is a possibility that the motor stops due to a loss of synchronism when the motor speed fluctuates immediately after the drive mode is switched to the self-controlled-drive.